1. Field of the Invention
This invention is directed to a modified freeze fracture imaging of a viscous surfactant mesophase.
2. Description of the Prior Art
Surfactants have an ability to self assemble into a wide variety of supramolecular structures such as micelles, bilayers, vesicles, liquid crystals, and emulsions. Self-assembly has become an important avenue for employing and fabricating nanostructures with useful electrical, optical, chemical and mechanical properties. Some factors that determine the specific type of aggregate formed by a surfactant in a solvent include the alkyl chain length, nature of solvent, surfactant concentration, temperature, salt concentration, and the presence of one or more cosurfactants. A wide range of aggregate structures and morphologies can be obtained in a mixed surfactant system. For example, aqueous mixtures of cationic and anionic surfactants form a variety of composition dependent microstructures including spherical micelles, worm-like micelles, vesicles and lamellar phases. The templating effect offered by the surfactant aggregates has been a proven tool for material synthesis. Mesoporous zeolites, porous polymers, biomimetic ceramics, and a range of other inorganic structures with different architecture can be synthesized within these templated systems. The various classes for templating have been classified as synergistic, transcriptive and reconstructive.
The ability to ‘visualize’ microstructures and examine morphologies is vital if these aggregates are to be used for guiding synthesis. One approach is to image in reciprocal space, using neutron, X-ray or light scattering, from which orientationally averaged morphologies can be obtained. While these techniques are very useful and look at a statistically large amount of sample, they rely on inversion from reciprocal space to real space to uncover morphologies, and are therefore model-dependent and sometimes not unique. The ability to generate direct images would serve as an extremely powerful complement to any scattering data. However, direct imaging of aggregates in solutions poses significant challenges. The length scales being probed fall into the range suitable for electron microscopy. Clearly, the sample cannot be exposed to the vacuum in an electron microscope, since all the solvent will evaporate. Surfactant aggregates are not electron dense, so staining with heavy metal salts is sometimes used. However, the addition of salts can have a dramatic impact on phase behavior. Therefore special techniques are required for artifact-free visualization using electron microscopy. These include cryogenic methods such as freeze frame transmission electron microscopy known as freeze fracture TEM (FFTEM) and cryogenic transmission electron microscopy (cryo-TEM). FFTEM is predicated upon rapid freezing of the sample, passing a fracture plane through aggregates, successful replication of the fracture surface and liftoff of the replica for direct visualization. This is a laborious technique, and is critically dependent on fracture planes passing through representative sections of the sample. What is observed is the surface topology along the fracture plane. In cryo-TEM, the sample is placed on a specially prepared microscope grid, thinned by blotting, and then vitrified by direct contact with the cryogen. If the sample has a large organic content, contact with the ethane cryogen causes dissolution, and restricts the ability to use normal cryo-TEM techniques for non destructive imaging. Additionally, highly viscous systems such as gels, cannot be thinned down by blotting. Freeze fracture direct imaging (FFDI) combines features of FETEM and cryo-TEM, arid alleviates many of these problems.
Freeze fracture direct imaging (FFDI) has been used to image microstructures present in a highly viscous four-component mesophase containing water, isooctane, AOT [bis(2-ethylexyl) sodium sulfosuccinate], and lecithin. As water is added to a fixed amount of a ternary solution of isooctane and the two surfactants, the microstructure evolves from a water-in-oil microemulsion, to a highly viscous columnar hexagonal, and then to multilamellar vesicles. Each of these microstructures is imaged directly. Previous small-angle neutron scattering measurements have identified the lamellar phase, but the FFDI technique demonstrates that these are onionlike curved multilamellar. structures rather than planar bilayers. Freeze fracture direct imaging expands the range of cryo-transmission microscopy to highly viscous, high-organic-content systems that typically have been difficult to visualize. One reference describing the FFDI method is Freeze Fracture Direct Imaging of a Viscous Surfactant Mesophase, Agarwal, et 2003